The invention relates to a cable having a substantially gastight metal tube receiving one or more optical conductors and a hydrogen-absorbent substance.
Optical conductors are in common use in the field of telecommunications. Data is generally conveyed optically at a wavelength of approximately 1300 nanometers or 1550 nanometers by optical fibers based on silica. Each fiber is protected by layers of a polymer material, and the protective layers are often covered with some other, pigmented, polymer.
A set of optical fibers can be assembled to form a ribbon. In which case, the material of the ribbon is also a polymer.
The individual optical fibers or the ribbon of optical fibers are disposed in a metal tube or in a plastics tube.
It is known that optical fibers must not be exposed to hydrogen because that gas degrades their transmission properties and possibly also their mechanical properties. The higher the partial pressure of hydrogen to which the fiber is subjected, the greater the degradation.
The hydrogen comes in particular from decomposition of the polymers constituting the coverings of the fibers or the material for assembling the fibers together into a ribbon. It can also come from decomposition of the filler material that is generally provided in the tube to hold the fibers in the tube and to prevent moisture from travelling in the event that the tube is punctured or degraded.
Such decomposition occurs naturally because of ageing.
When the tube is made of a plastics material, the permeability of the material enables hydrogen to be diffused. When the tube is made of metal or of some other non-porous material, hydrogen remains confined inside the tube, and the optical properties of the fibers are thus progressively degraded.
To solve that problem, one solution consists in using a tube made of stainless steel so as to avoid giving rise to hydrogen-generating corrosion, and in providing a filler material that has hydrogen-absorbent properties. For example, the filler material may be a polymer, in particular an unsaturated polymer with which a catalyst such as palladium is generally mixed.
Stainless steel and such a mixture of a polymer with a catalyst constitute materials that are costly.
The invention remedies that drawback. It makes it possible to omit stainless steel and such a polymer mixture.
In the invention, the inside face of the metal tube of the cable is covered, at least in part and preferably substantially totally, with a layer of a catalyst substance facilitating hydrogen absorption by the substance provided for this purpose, said layer itself being covered, at least in part and preferably substantially totally, with at least one layer of the hydrogen-absorbent substance. In an embodiment, the hydrogen-absorbent substance constitutes the filler material. In another embodiment, the hydrogen-absorbent substance forms merely a layer in contact with the catalyst.
The invention results from the observation that it is not essential to mix the catalyst with the hydrogen-absorbent substance which is generally organic. It is merely necessary to deposit the catalyst in contact with the hydrogen-absorbent substance, the most suitable place being the inside surface of the tube. With such a configuration, the catalyzing effect is, in principle, less effective that when the catalyst is mixed with the filler material because, with the invention, the specific surface area of the catalyst that is in contact with the hydrogen-absorbent substance is smaller than the corresponding specific surface area when the catalyst is mixed with the filler material. Nevertheless, it has been observed that, in spite of this handicap, the hydrogen absorption effect remains satisfactory.
However, it is possible to increase to the specific surface area in contact with the hydrogen-absorbent substance, i.e. to increase the effectiveness of the catalysis, by imparting a selected amount of roughness to the surface of the catalyst. For example, this may be achieved by spraying on the catalyst in granular form, or by providing grooves, in particular longitudinal grooves, in the surface of the catalyst.
The tube can be manufactured from a metal strip whose inside surface is covered with the catalyst substance, the strip being subjected to forming so as to impart the shape of a tube to it, the longitudinal edges of the strip of metal being welded together once the optical conductors and the filler material have been fed into the formed tube.
In a variant, instead of welding the edges together, one longitudinal margin of the metal strip is applied against the other longitudinal margin and the two margins are glued together. When the hydrogen-absorbent substance constitutes a layer in contact with the catalyst, the gluing may be achieved by means of said layer which may be constituted by copolymers, for example. With this variant, the tube is less gastight than when it is welded. However, the leakage is insufficient to enable hydrogen to escape naturally, and it is therefore necessary to provide hydrogen-absorbent substance. But, in this case, the hydrogen absorption need not be as effective as when the tube is gastight.
The catalyst is preferably chosen such that it protects the metal tube against corrosion. Thus, it is not necessary to use stainless steel. It is also possible to cover the outside surface of the tube with the same material as that covering its inside surface.
In an embodiment, the metal tube is made of steel, in particular mild steel, and the covering on the inside of the tube is made of nickel and/or of chromium which performs the combined functions of catalyst and of protective covering for protecting the steel against corrosion. In which case, it is advantageous to nickel plate and/or to chromium plate both faces (inside and outside) so as to protect the tube fully against corrosion.
For example, the hydrogen-absorbent substance may contain a polymer which is chosen from the group formed by ethylene vinyl acetate (EVA) and by ethylene ethyl acrylate (EEA). For example, it may be advantageous to choose as the metal for making the tube a commercially available steel, in particular a mild steel, covered with a metal layer e.g. of nickel and/or of chromium, itself covered with a layer of a polymer such as EVA and/or EEA, which layer of polymer may advantageously be used to enable the tube to be glued together when it is formed.
The hydrogen-absorbent substance contains an organic material which is preferably a polymer that has double bonds (dienes) which open in the presence of the catalyst and which facilitate fixing the hydrogen on the open double bonds. The advantage with this type of polymer is that no water is produced by hydrogen being transformed into water which is trapped in the cable, in particular when the cable is substantially hermetically gastight, and thus the risk of optical fiber fatigue in the presence of water is smaller. For example, the filler material may be based on silicone or on a polyolefin, and, in the latter case, it is a polybutadiene or a polyethylene glycol or a combination of these two substances.
When the hydrogen-absorbent substance is based on a polymer, it is possible to use as the catalyst at least one of the metals in the platinum class, i.e. one of the elements in group VIII, periods 4, 5, and 6, of the periodic table of the elements, and preferably, in addition to nickel: cobalt, palladium, and chromium.